Andreas Lubitz put the plane into a descent after the captain had left the cockpit, Marseille prosecutor Brice Robin told reporters.

Evidence from the black box flight recorder suggests he then refused to open the cockpit door to the captain.

The captain knocked on the door and asked to be let in and there was no answer from the co-pilot. Later there are the sounds of attempts to break down the door, the prosecutor said.

The prosecutor said the cockpit voice recorder gave information from the first 30 minutes of the flight.

For the first 20 minutes the two pilots talked in a normal fashion and were as courteous as two normal pilots would be.

Then the captain is heard asking the co-pilot to take over and the sound of a chair being pushed back and a door being closed is heard

Asked about Mr Lubitz's ethnicity, Mr Robin said: "He was a German national and I don't know his ethnic background.

"He is not listed as a terrorist, if that is what you are insinuating.''

Pressed on the co-pilot's religion, he said: "I don't think this is where this lies. I don't think we will get any answers there.''

Mr Robin said black box recordings showed that Mr Lubitz "was breathing normally, it wasn't the breathing of someone who was struggling''.

Speaking about whether the passengers realised what was happening, Mr Robin said: "I think the victims only realised at the last moment because on the recording we only hear the screams on the last moments of the recording.''

He added: "I believe that we owe the families the transparency of what the investigation is pointing to and what is going on, we owe it to them to tell them what happened.

"The families have been informed of everything I just told you.''

It was assumed that the captain had gone to the toilet, leaving the co-pilot in charge of the plane, the prosecutor said.

Mr Robin went on: "The co-pilot uses the flight monitoring system to start the descent of the plane. This can only be done voluntarily, not automatically.

"We hear several cries from the captain asking to get in. Through the intercom system he identifies himself - but there is no answer. He knocks on the door and asks for it to be opened - but there is no answer.''

Mr Robin said that after entry to the cockpit was denied, the sound of breathing from inside the cockpit was heard and this sound carried on until the moment of impact. "The co-pilot was still alive at this point,'' Mr Robin said.

He added that there was no answer from the cockpit to communication with ground controllers or from other aircraft in the area.

The cockpit voice recorder then shows that there were alarm signals going off which indicated to all those on board the proximity of the ground.

Acquaintances in the German town of Montabaur, in the Rhineland-Palatinate region of western Germany, said Mr Lubitz had showed no signs of depression when they saw him last autumn.

"He was happy he had the job with Germanwings and he was doing well,'' said a member of a glider club who watched him learn to fly.

Peter Ruecker said: "He was happy he had the job with Germanwings and he was doing well. He gave off a good feeling.''

Mr Lubitz had obtained his glider pilot's licence as a teenager and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Mr Ruecker said.

He described the co-pilot as a "rather quiet'' but friendly young man.

Noises of someone trying to break down the cockpit door are then heard. Finally the sound of an impact is heard.

Mr Robin said the plane may have glided before the moment of impact.

He said there was no distress signal, no Mayday and no answer despite numerous calls to the plane.

Mr Robin said: "The interpretation of the cockpit voice recorder evidence is that the co-pilot voluntarily refused to open the cockpit door to the captain.''